EECB 423 Data
Communication and Network
Lecture 5
ISO OSI RM
OSI Model
Open System Interconnect
(OSI)
Reference Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
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Why Layer Model
•
•
•
•
•
Reduces complexity
Standardizes interfaces
Facilitates modular engineering
Ensures interoperate technology
Accelerates the evolution of network
technology
• Simplifies teaching and learning
OSI Model - History
• First studied by Honeywell Information
Systems group headed by Mike Canepa with
Charlie Bachman as principal technical
member
• The purpose was to determined a structured
distributed communication architecture for
database and distributed database application.
• 1977 proposed a seven layer architecture
known as DSA (Distributed System
Architecture). Mainly based on IBM’s SNA.
Copyrighted by William Stalling
2
OSI Model - History
• 1977 British Standards Institute proposed to
ISO that a standard architecture was needed to
define the infrastructure for distributed
computing
• ISO subcommittee on OSI (Technical
committee 97, subcommittee 16) is formed.
ANSI was charged to develop the proposal
• 1978 Honeywell group proposed DSA to ANSI.
In June 1979 it was accepted as a standard
with minor refinement.
Copyrighted by William Stalling
OSI Model – Physical Layer
o deals with mechanical, electrical and
procedural interfacing
o specifies cables, connectors, and other
components
o transmits raw information over
communication channel
o establishes, maintains, and disconnects
physical links
3
OSI Model – Data Link Layer
o provides reliable transfer of data
o Packs data (packets) into frames
o adds bits for error detection/correction
o manages access to and use of the
channel
o solve problems caused by lost, damaged,
and duplicate frames
OSI Model – Data link layer
(Continue)
o sends acknowledgments*
o adds flags to indicate beginning and end
of message
o connectionless or connection oriented
services
o IEEE MAC and LLC support
4
OSI Model – Network Layer
o establishes, maintains and terminates
connections*
o determines how packets are routed
o divides transport messages into packets
and reassembles them*
o performs congestion control, flow control
OSI Model – Network Layer
(Continue)
o provides virtual circuit or datagram
services
o recognizes message priorities
o sends messages in proper order*
o handles internetworking
5
OSI Model – Transport layer
o establishes reliable end-to-end transport
session (error detection and recovery),
once path has been established
o fragmentation of message into packets (if
not handled by layer 3)
o multiplexing of several sessions from
same source and all going to same
destination (logical)
OSI Model – Session
o establishes and controls systemdependent issues
o establishes and terminates connections
o accounting service
o authentication of users
o controls dialogue, organizes and
synchronizes
6
OSI Model – Presentation
o data encryption, security, compression
and code conversion
o make sure data is encoded in standard
form (ASCII)
o handles pass-through of services from
session to application layer
OSI Model – Application
o login, password check
o agreement on semantics for information
exchange
o file transfer, access and management
o message handling, email
o job transfer and manipulation
o directory service
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Information Flow
Application A
Application B
Application
Layer
Application
Layer
Presentation
Layer
Presentation
Layer
Session
Layer
Session
Layer
Transport
Layer
Communication Network (Subnet)
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Electrical and/or Optical Signals
Inter-Device Flow
n-PDUs
n
n
entity
entity
PDU = Header + SDU
8
n+1
entity
n+1
entity
n-SDU
n-SDU
n-SAP
n-SAP
n-SDU
H
n entity
n entity
n-SDU
H
n-PDU
Application A
Application
Layer
data
Session
Layer
sh
Message Segment
Transport
Layer
data
Transport
Layer
th
Packet (Datagram)
Network
Layer
Physical
Layer
Presentation
Layer
ph
data
Session
Layer
Application
Layer
ah
data
Presentation
Layer
Data Link
Layer
Application B
data
data
Network
Layer
nh
Frame
dt
data
bits
dh
Data Link
Layer
Physical
Layer
Figure 2.9
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Quiz 1
• Dialog Management: Half Duplex
Communication
• Topology
• DNS (Domain Name Service)
• Logical Connection
• Big Endian / Little Endian
Quiz 2
• Encryption/Decryption
• Signaling Protocol
• SNMP
• Re-ordering
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GAME: Match the Layer
• ASCII
• EBCDIC
• Big Endian / Little Endian
• Compression & Decompression
• Encryption/Decryption
A story of endian
• In big-endian architectures, the leftmost bytes
(those with a lower address) are most significant.
In little-endian architectures, the rightmost bytes
are most significant
• ‘UNIX’ (2 byte words) store as:
Big Endian: UNIX
Little Endian: NUXI
• Big Endian: Mainframe computer including IBM
• Little Endian: Modern computer including PC
• Bi-endian: PowerPC
• Apply to bit ordering as well
• Bits & Byte re-ordering: NUXI process
Source: Webopedia
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GAME: Match the Layer
• Modulation
• Signaling Protocol
• Pin Assignment
• Signal Multiplexing
• Topology
GAME: Match the Layer
• Acknowledgement
• Collision Detection
• CRC
• SDLC, HDLC, PPP
• Switches
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GAME: Match the Layer
• DNS (Domain Name Service)
• X.500
• VT100
• SNMP
GAME: Match the Layer
• Synchronization
• Dialog Management: Half Duplex
Communication
• Command Quarantining
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GAME: Match the Layer
• Re-ordering
• Logical Connection
• Logical Multiplexing
• Handshaking
GAME: Match the Layer
(a)
n-PDU
(b)
Segmentation
Reassembly
n-SDU
n-SDU
n-PDU
n-PDU
n-PDU
Blocking
n-SDU
n-SDU
n-PDU
n-PDU
n-PDU
Unblocking
n-SDU
n-SDU
n-SDU
n-SDU
n-PDU
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GAME: Match the Layer
• Routing
• LAN-WAN Connection
• Congesting Control
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